A single photon avalanche detector (SPAD) is based on a PN junction device biased beyond its breakdown region. A high reverse bias voltage generates a sufficiently large electric field such that a single charge carrier introduced into a depletion layer of the PN junction device can cause a self-sustaining avalanche. This charge carrier may be released by the impact of a photon (impact ionization). The SPAD may be quenched, allowing the device to be reset to detect further photons.